Process for the preparation of thiourea

ABSTRACT

The present invention relates to an improved process for the preparation of thiourea. The object of the present invention is to provide an improved process for the preparation of Thiourea using Calcium Cyanamide, carbon dioxide and hydrogen sulphide. The process of the present invention comprises passing a mixture of Carbon dioxide ad hydrogen sulphide into a slurry formed by addition of major part of calcium cyanamide charge into water using constant stirring, maintaining alkaline pH at a temperature ranging between ambient to 80° C., stopping the addition of hydrogen sulphide, continuing the slow passing of carbon dioxide and addition of remaining part of calcium cyanamide charge and retaining the reaction mass to complete the secondary reactions to form the product for a period ranging from 2-5 hours and continuing passing of carbon dioxide at an increased rate for effecting decomposition of Ca(SH) 2  for a period of 1.6 to 6 hours, stopping the addition of carbon dioxide separating thiourea solution, treating the separated solution with activated carbon, removing the carbon by conventional methods, separating the product formed by conventional methods and drying the product at a temperature between 50-70° C. to obtain the product.

[0001] This invention relates to an improved process for the preparationof thiourea.

[0002] Thiourea finds application in manufacture of amino resins,herbicides, fungicides, insecticides, plant growth regulators,photographic paper. It finds use in electrochemical processes,pharmaceutical industries, textile processing, hydrometallurgy, rubberindustry and petroleum industry.

[0003] In the prior art thiourea is presently produced essentially bythe following methods.

[0004] (a) from ammonium sulphocyanide by isomerisation in moltencondition:

[0005] (b) from Calcium Cyanamide and ammonium sulphide:

[0006] (c) from Calcium Cyanamide and calcium hydrosulphide:

[0007] (d) from Calcium Cyanamide and hydrogen sulphide and subsequentconversion by method (c).

[0008] The above mentioned processes are briefly described herein-below.

[0009] (a) The isomerisation of ammonium sulphocyanide involves meltingthe ammonium thiocyanate and keeping upto temperature level of 140-170°and dissolving the thiocarbamide from the cooled ground melt.

NH₄SCN→CS(NH₂)₂

[0010] (b) The preparation by the reaction between calcium cyanamide andammonium sulphide involves addition of the solution of the latter at25-80° C. and blowing off ammonia, or in the presence of an ammoniumsalt such as carbonate, sulphate or oxalate.

[0011] (c) The calcium hydrosulphide method involves addition ofcyanamide to a solution of calcium hydrosulphide such that thetemperature is controlled around 70-80° C. The reaction is:

2CaCN₂+Ca(SH)₂+6H₂O→3Ca(OH)₂+2CS(NH₂)₂

[0012] The product is isolated from the reaction mass by filtration,concentration at temperatures below 98°, filtration to remove calciumsalts and crystallisation.

[0013] (d) The method using reaction between calcium cyanamide andhydrogen sulphide consists of passing a current of hydrogen sulphide ina suspension of calcium cyanamide with good agitation and avoiding unduerise in temperature.

[0014] The reaction is:

CaCN₂+3H₂S→Ca(SH)₂+CS(NH₂)₂

[0015] The Ca(SH)₂ is converted into thiourea by further addition ofcalcium cyanamide according to the following reaction.

2CaCN₂+Ca(SH)₂+6H₂O→2CS(NH₂)₂+3Ca(OH)₂

[0016] Besides the above the other methods include the following.

[0017] (a) Treatment of CaCN₂ with H₂S in the presence of small amountof water, methanol, ethylacetate, aniline or esters, hydrocarbons likebenzene or their halogen derivatives.

[0018] (b) Reacting together CaCN₂ and alkaline earth sulphide inpresence of water and carbon dioxide in a ball mill.

[0019] (c) Reacting alkaline earth cyanamide such as CaCN₂ with SO₂ orCO₂ till calcium is removed and making alkaline with ammonia and furtherreaction with H₂S and completion of the reaction by heating.

[0020] (d) Converting CaCN₂ by hydrolysis to Ca(HCN₂)₂ and passing H₂Sand subsequent treatment to remove calcium by CO₂ containing gases.

[0021] (e) Treating CaCN₂ in water with sulphuric acid at lowtemperatures, making ammoniacal and treatment with H₂S at lowtemperature and acidifying and cooling.

[0022] (f) By reaction between cyanamide and Na₂S in aqueous phosphateor borate buffers at pH 6-10 at 25° C.

[0023] (g) By simultaneous action of CO₂ and H₂S in high H₂S ratios onaqueous suspension of calcium cyanamide.

[0024] (h) Reaction of As₂S₃ ore in presence of sulphur with CaCN₂ suchthat the ratio of Nitrogen and S are in the ratio 1:1.

[0025] The above methods suffer from the following draw-backs:

[0026] 1. The isomerisation methods given conversions only upto amaximum of 25% at the most and isolation involves tedious methods.

[0027] 2. Ammonium sulphide is not commercially available and the methodinvolving its use produces ammonia. Impurities like sulphate are alsopresent.

[0028] 3. The method using calcium hydrosulphide or hydrogen sulphideproduces lime and its removal completely is not possible. Impuritieslike calcium trithiocarbonate and calcium sulphocyanide are formed. Thisrequires discarding of mother liquors where thiourea is lost. Formationof calcium hydroxide is detrimental to the product.

[0029] 4. The reaction between calcium cyanamide and hydrogen sulphidesuffers from the following counts:

[0030] a) The consumption of hydrogen sulphide is high and producesequimolar quantity of calcium hydrosulphide.

[0031] b) The calcium hydrosulphide on reaction with calcium cyanamideproduces calcium hydroxide which is difficult to remove as stated in (3)above.

[0032] 5. The process which use simultaneous use of CO₂ and H₂S consumea large amount of expensive and toxic H₂S and no recovery of the unusedH₂S is indicated. The auto-genous temperature rise may give rise tosulphur components other than thiocarbamide.

[0033] 6. Methods involving use of As₂S₃ and Na₂S are of limited use andapplications due to very stringent reaction conditions.

[0034] The object of the present invention is to provide an improvedprocess for the preparation of Thiourea using Calcium Cyanamide, carbondioxide and hydrogen sulphide.

[0035] Accordingly the present invention provides an improved processfor the preparation of thiourea which comprises passing a mixture ofcarbon dioxide and hydrogen sulphide into a slurry formed by addition ofmajor part of calcium cyanamide charge into water under constantstirring, maintaining alkaline pH at a temperature ranging betweenambient to 80° C., stopping the addition of hydrogen sulphide,continuing the slow passing of carbon dioxide and addition of remainingpart of calcium cyanamide charge and retaining the reaction mass tocomplete the secondary reactions to form the product for a periodranging from 2 to 5 hours and continuing passing of carbon dioxide at anincreased rate for effecting decomposition of Ca(SH)₂ for a period of1.6 to 6 hours, stopping the addition of carbon dioxide, separatingthiourea solution by conventional methods, treating the separatedsolution with activated carbon, removing the carbon by conventionalmethods, separating the product formed by conventional methods anddrying the product at a temperature between 50 to 70° C. to obtain theproduct.

[0036] In an embodiment of the present invention the pH of the reactionmixture maintained may be at a value 8.0 to 11.0.

[0037] In another embodiment of the present invention the rate ofaddition of carbon dioxide may range between 2.0 to 3.33 gm moles perhour per kg charge of calcium cyanamide during the simultaneous passageof hydrogen sulphide.

[0038] In another embodiment of the present invention the rate ofaddition of hydrogen sulphide may range between 2.6 to 3.4 gm moles perhour per kg. charge of calcium cyanamide.

[0039] In another embodiment of the present invention, the time ofaddition of 85-90% of total charge of calcium cyanamide may rangebetween 1.8 to 2.2 hour.

[0040] In another embodiment of the present invention the time ofaddition of 10-15% of total charge of calcium cyanamide may range btween½ to 1 hour.

[0041] In another embodiment of the present invention the reactionmixture after the addition of 15% of total charge may be kept under asmall feed of carbon dioxide ranging from 0.32 to 1.0 gm mole per hourper kg charge of calcium cyanamide.

[0042] In another embodiment of the present invention the reaction massmay be retained for a period ranging from 2 to 5 hours after theaddition of calcium cyanamide is complete.

[0043] In yet another embodiment of the present invention, the rate ofaddition of carbon dioxide for decomposition of Ca(SH)₂ may range from0.8 to 2.5 gm moles per hour per kg charge of calcium cyanamide.

[0044] In a feature of the present invention the calcium carbonate isremoved by conventional methods like filtration and the product isrecovered by treating the filtrate with activated carbon at 60 to 80°C., and subsequent evaporation under reduced pressure andcrystallisation at 10-15° C. The pressure employed may range from 55 to65 mm Hg at a temperature of 50 to 70° C.

[0045] The crystals are filtered and washed with water at 0 to 5° C. anddried at 50-80° C. The washings of crystals can be recycled toevaporator.

[0046] The conversion of thiourea based on calcium cyanamide is 85-92.5%and the yields of the product range from 65-75% with purities ranging95-98% based on a single crop. Selectivity based on hydrogen sulphidefed is 72-72.5% and 100% on hydrogen sulphide consumed in all cases.

[0047] In another feature of the present invention the process can beused even with mother liquors containing thiourea obtained aftercrystallising it. The quality of the product is comparable with thatobtained from the customary process once through.

[0048] The invention is illustrated by the following examples, which maynot be construed to limit the scope of this invention in any mannerwhatsoever.

EXAMPLE 1

[0049] In a five necked cylindrical glass reactor of 3 liter capacity,2.4 liter of water was filled and was kept agitated vigorously. It waskept in water bath. Hydrogen sulphide and carbon dioxide were fed at aregulated rate and 600 gm of calcium cyanamide (CaCN₂ 42.0% CaO 34.06%)was added slowly. Hydrogen sulphide was passed at a rate of about 57.5gm per hour and carbon dioxide was passed at a rate of 50.0 gm per hourand were measured by suitable flow meters. About 85% of total calciumcyanamide was added during 130 minutes and total hydrogen sulphidepassed was 126.4 gm (18% excess over theory). Carbon dioxide passedduring this period was 133.8 gm. During the above reaction the pH variedfrom 8.6-9.7 and the temperature varied from 29-56° C. The hydrogensulphide was stopped but carbon dioxide was passed at a rate of 21.5 gmper hour and the balance calcium cyanamide was added. The total durationwas 170 minutes. During passing the carbon dioxide gas the pH rangedfrom 8.6-9.1 and the temperature ranged between 32-35° C.

[0050] The reaction mass was then treated with 98 gms of carbon dioxidefor 100 minutes till sulphide was absent by usual tests.

[0051] The temperature was kept between 40-65° C. during this operation.

[0052] The final mass on filtration under suction and washing with 1.25liter hot water yielded 3.73 liter of filtrate. On analysis of thefiltrate and the solids the conversion to thiourea was found to be 85.0on CaCN₂ content of charge.

[0053] The filtrate on concentration in vacua to 400 ml volume oncooling to 10° C. yielded 145.5 gm (yield 69.05%) crystals which assayedto 98.0% thiourea. The mother liquor was discarded. The selectivitybased on hydrogen sulphide fed was 72.0%.

EXAMPLE 2

[0054] Six hundred grams of calcium cyanamide (CaCN₂ 42.0% CaO 34.06%was charged into 2.4 liter water and treated with 129.0 gm of hydrogensulphide (20.6% excess over theory) and also 150 gm of carbon dioxidefor 2.25 hours followed by 15 gm of carbon dioxide for 2 hours andfinally by 104.5 gm carbon dioxide for 2.25 hours till sulphide wasabsent:

[0055] Further processing such as filtration under suction and washingwith 1.15 litre of hot water yielded a filtrate of 3.61 liter. Onanalysis the conversion was found to be 90.76% on CaCN₂ content.

[0056] The temperature ranged from 21°-60° and pH 8.8-9.6.

[0057] The filtrate on treatment with 55 gm of decolorising charcoal at60°-80° C. for 30 minutes, filtration, concentration in vacua to 450 mland cooling to 15° C. yielded 158 gm of thiourea (66.0% yield) and ofpurity 98.4% on assaying selectivity based on hydrogen sulphide fed was75.4%.

EXAMPLE 3

[0058] Six hundred gms of calcium cyanamide (CaCN₂ 42.0%, CaO 34.06%)was charged into a mixture of 280 ml of mother liquor containing 31.28gms of thiourea and 18.95 gms of the solids from a previous batch and2.120 ml water on treatment with 132.8 gm of hydrogen sulphide (excess24.1%) and 167 gm of carbon dioxide for 155 minutes, followed by 8.1 gmcarbon dioxide for 55 minutes and 129.5 gm of carbon dioxide for 160minutes till sulphide was absent yielded a filtrate of 3.25 liter onprocessing as usual. The conversion to thiourea on CaCN₂ was found to be90.47% as analysis of filtrate and filter cake.

[0059] The pH varied from 8.8-9.7 and the temperature ranged from 28-65°C. The filtrate on concentration under vacua to 375 ml and cooling to10° C. yielded crystals of 175.0 gm of thiourea (yield 73.1%) assaying95.6%. The selectivity based on hydrogen sulphide fed was 73.0%.

EXAMPLE 4

[0060] In metallic reactor of 250 litre capacity with jacket, stirrer,gas spargers, solid feeding arrangement, continuous pH measuring andtemperature measuring devices, 160 liter of water was kept circulated bya powerful pump. Forty Kg of calcium cyanamide (CaCN₂ 46.3%, CaO 35.88%)was added slowly with 8.0 kg of hydrogen sulphide (9.89% excess) and10.6 kg carbon dioxide for 120 minutes. About 85% of total calciumcyanamide was added during the above period.

[0061] The feed of hydrogen sulphide was stopped, balance of 15% ofcalcium cyanamide was added over a period of 40 minutes and 2.3 kg ofcarbon dioxide was passed for 240 minutes.

[0062] Then 8.5 kg of carbon dioxide was passed for 6 hours for sulphideremoval. Cooling water was kept running in the jacket. The pH rangedfrom 8.5-9.3 and temperatures 31-42° C.

[0063] Filtration and washing with 90 liter water in a vacuum rotarydrum yielded 245 liter of filtrate. The conversion to thiourea was 92.5%including the thiourea in filter cake (61 kg wet).

[0064] This filtrate along with 10 liter of crystal wash from previousbatch containing 600 gm thiourea on evaporation under vacuum to 36 literand crystallising at 12° C. and washing with ice-cold water at 0-5° C.yielded 12.2 kg of thiourea (69.3% yield). Purity on assaying was foundto be 97.6%.

[0065] Selectivity based on hydrogen sulphide feed was 91.0%.

EXAMPLE 5

[0066] A 40 kg quantity of calcium cyanamide assaying 37.6% CaCN₂ with160 litre water, 7.4 kg (110 minutes) and total 17.1 kg carbon dioxidewas passed at 8.5 kg for 135 minutes, 1.8 kg in 270 minutes and 6.0 kgin 360 minutes for reaction, retention and sulphide removalrespectively, in the apparatus previously described, indicated aconversion of 88.2% to thiourea on CaCN₂, a yield of 68%, purity of97.8% and a selectivity on hydrogen sulfide of 86.4%.

[0067] The main advantages of the present invention are:

[0068] 1) Very good yield in the range of 65-75% of Thiourea.

[0069] 2) Isolation of the thiourea formed by conventional methods.

[0070] 3) Elimination of chemicals like (NH₄)₂S which are notcommercially available.

[0071] 4) Selectivity based on the hydrogen sulphide fed in the range of72-91% and 100% on the basis of hydrogen sulphide consumed.

[0072] 5) Product obtained by the improved process is of very highpurity in the range of 95.6-98%.

[0073] 6) Conversion based on calcium cyanamide in the range of 85-92.5%of thiourea.

[0074] 7) Calcium cyanamide of low assay could also be used to producethiourea of good quality.

[0075] 8) The reaction is carried out with very low excess of hydrogensulphide which prevents the formation of other sulphurcompounds.

[0076] 9) The excess of hydrogen sulphide used is between 10-25% overtheory as against 50-100 which is common practice.

[0077] 10) The addition of calcium cyanamide being slow and controlleddoes not allow the formation of by-products like dicyanamide.

[0078] 11) The conditions of the reaction are more conducive toformation of SH⁻ ions which are essential for the formation of thiourea.

[0079] 12) The reaction schemes involved using calcium cyanamide, carbondioxide and hydrogen sulphide are simple and yet novel.

We claim:
 1. An improved process for the preparation of thiourea whichcomprises passing a mixture of carbon dioxide and hydrogen sulphide intoa slurry formed by addition of major part of Calcium cyanamide chargeinto water under constant stirring, maintaining alkaline pH at atemperature ranging between ambient to 80° C., stopping the addition ofhydrogen sulphide, continuing the slow passing of carbon dioxide andaddition of remaining part of calcium cyanamide charge and retainingreaction mass to complete the secondary reactions to form the productfor a period ranging from 2 to 5 hours and continuing passing of carbondioxide at an increased rate for effecting decomposition of Ca(SH)₂ fora period of 1.6 to 6 hours, stopping the addition of carbon dioxide,separating thiourea solution by conventional methods, treating theseparated solution with activated carbon, removing the carbon byconventional methods, separating the product formed by conventionalmethods and drying the product at a temperature between 50 to 70° C. toobtain the product.
 2. An improved process as claimed in claim 1 whereinthe pH of the reaction mixture is maintained at values 8.0 to 11.0. 3.An improved process a claimed in claim 1 and 2 wherein the rate ofaddition of carbon dioxide ranges between 2.0 to 3.33 gm. moles per hourper kg. charge of calcium cyanamide.
 4. An improved process as claimedin claims 1 to 3 wherein the rate of hydrogen sulphide ranges between2.6 to 3.4 gm. moles per hour per kg. charge of calcium cyanamide.
 5. Animproved process as claimed in claims 1 to 4 wherein the rate of carbondioxide addition during retention after the addition of calciumcyanamide completed ranges from 0.32 to 1.0 gm mole per hour per kg ofthe charge of the calcium cyanamide.
 6. An improved process as claimedin claims 1 to 5 wherein the rate of carbon dioxide addition fordecomposition of Ca(SH)₂ ranges from 0.8 to 2.5 gm moles per hour per kgof the charge of calcium cyanamide.
 7. An improved process as claimed inclaims 1 to 6 wherein the temperature of the reaction mixture ismaintained in the range of ambient to 80° C.
 8. An improved process forthe preparation of thiourea as substantially described hereinbefore withreference to the examples.